The invention relates to a fuel supply system for an internal combustion engine comprising a fuel tank, a fuel pump supplying the fuel from the tank under pressure to the engine and a fuel processing unit, in which at least two fuel fractions are produced which are supplied to the engine depending on the engine operating conditions, and to a method of supplying fuel and the fuel fractions to an internal combustion engine.
The fuels currently available for operating internal combustion engines, e.g. for motor vehicles such as trucks, passenger cars, buses, constitute a compromise between, on the one hand, restrictions on the part of the fuel manufacturers, e.g. on account of different crude oil grades, manufacturing processes, costs and energy input, and, on the other hand, partly conflicting requirements with respect to the internal combustion engines, such as, for example, reliable cold starting even at extremely low temperatures, low exhaust-gas and evaporative emissions, low consumption, high knock rating even in super-charged engines, prevention of deposits, avoidance of corrosion, low sulphur content, smooth engine running and a high degree of safety. In order to be able to adapt the existing fuels to the requirements of the internal combustion engine more effectively, fuel supply systems of the type mentioned at the beginning are used.
For example, GB 2 209 796 A discloses a fuel-fractionating device which is connected to a fuel tank and separates fractions of different quality from the fuel. These fuel fractions are stored in separate fuel tanks, which are connected to an internal combustion engine via separate fuel lines. One fuel tank is heated by the exhaust gases of the internal combustion engine by means of a heating line. Arranged in the fuel lines are valves which regulate the fuel quality of the fuel fed to the internal combustion engine as a function of the operating state of the internal combustion engine and/or as a function of the fill level of the fuel tank. A microprocessor may be provided in order to regulate a fuel injection system, the quality of the fed fuel and the ignition timing.
DE 197 34 493 C1 discloses a fuel supply system of the type mentioned at the beginning which has a fuel tank for liquid fuel. In addition, a fuel-fractionating device is provided which, on the inlet side, receives the fuel from the fuel tank and produces therefrom a low-boiling fuel fraction and a higher-boiling fuel fraction. The fuel supply system includes a separate accumulator for each fuel fraction. These accumulators receive the respective liquid fuel fraction from the fuel-fractionating device, store them and make them available to the internal combustion engine. Furthermore, the fuel supply system has a main fuel pump which, on the suction side, is connected, via a switchover valve, either to the fuel tank containing the fuel or to the accumulator containing the higher-boiling fuel fraction. On the pressure side, the fuel pump is connected to a first inlet of a switchover valve. Connected to the second inlet of this switchover valve is the pressure side of an auxiliary fuel pump, which is connected on the suction side to the accumulator containing the low-boiling fuel fraction. The outlet side of the switchover valve is connected to the internal combustion engine. In order to be able to make both, the fuel and the individual fuel fractions, available to the internal combustion engine at the injection pressure, two separate fuel pumps are used. However, such a setup is relatively complicated and expensive.
EP 0 060 976 A1 discloses a fuel supply system for an internal combustion engine, wherein a fuel pump receives fuel from a fuel tank and supplies the fuel on one hand to an injection valve and, on the other, to a processing apparatus. In the processing apparatus, a low boiling fuel fraction is evaporated and the fuel vapors are then condensed. The condensed fuel fraction is supplied to an auxiliary tank, which is also in communication with the injection valve. During startup of the engine, the fuel fraction of the auxiliary tank is supplied to the engine. Since the fuel line system is a closed system, the injection pressure is also present in the auxiliary tank.
GB 2 330 176 A discloses a fuel supply system, wherein the fuel from the fuel tank is supplied by a first pump to an evaporator unit. In the evaporator unit, a low boiling fuel fraction is evaporated. The higher boiling remaining fuel fraction is supplied, during normal engine operation to the internal combustion engine by means of a second fuel pump. A third fuel pump supplies the evaporated lower boiling fuel fraction to a condenser in the form of a pressure storage container wherein the fuel vapors are condensed. The low boiling fuel fraction is subjected in this pressure storage container to a higher injection pressure than the higher boiling fuel fraction. For startup of the engine, the low boiling fuel fraction is supplied to the engine.
The present invention deals with the problem of reducing the cost, which is required in order to make the fuel fraction and the fuel available to the internal combustion engine at the same pressure.
In a fuel supply system for an internal combustion engine having a fuel tank for liquid fuel, a fuel pump which draws fuel from the fuel tank and pressurized the fuel to an injection pressure at which the fuel is made available to the internal combustion engine, a fuel-fractionating device which produces at least one liquid fuel fraction from the fuel, and an accumulator which receives the liquid fuel fraction from the fuel-fractionating device, stores it and makes it available to the internal combustion engine, the fuel fraction made available and the fuel made available being fed to the internal combustion engine by the fuel supply system as a function of demand, the accumulator is a pressure accumulator and includes pressure-generating means for pressurizing fuel fraction in the pressure accumulator to the injection pressure.
Due to the accumulator being designed as a pressure accumulator, the fuel fraction contained therein can be pressurized to the fuel injection pressure. In this way, less complicated pressure-generating means can be used for generating the injection pressure. In particular, pressure-generating means, which are already present in the internal combustion engine or in its peripheral area, may be used.
In accordance with the invention, a bellows is arranged in the pressure accumulator, which is connected to a pressure source, preferably to the already existing fuel pump, which can apply the injection pressure to the accumulator. Since the fuel pump delivers the injection pressure anyway, no additional measures need be taken in such an embodiment.
In an alternative embodiment, the fuel-processing or fractionating apparatus includes a vapor pump, which is connected with its suction side to an evaporation region and with its pressure side to a condensation region. In such an embodiment, the existing vapor pump is expediently used for generating the injection pressure and is connected in an appropriate manner to the pressure accumulator. In this embodiment, too, no additional, complicated measures are required.
In accordance with an especially advantageous embodiment, a condensation chamber of the fuel-processing apparatus forms the accumulator or the pressure accumulator. This means that this condensation chamber is dimensioned not only for a condensation pressure but also for the injection pressure. In addition, due to the condensation chamber being designed as an accumulator, additional lines and the sealing problems associated therewith are avoided.
In another advantageous embodiment of a fuel supply system of the type mentioned at the beginning, a condensation chamber of the fuel-fractionating device may be of cylindrical, in particular circular-cylindrical, design, whereas an evaporation chamber of the fuel-fractionating device is of annular design and is arranged concentrically and coaxially to the condensation chamber and annularly encloses the latter. This design of condensation chamber and evaporation chamber results in an especially favorable heat exchange between the chambers, so that an external heat supply for assisting the evaporation in the evaporation chamber may not be needed. Corresponding cooling of the condensation chamber may likewise be omitted.
The problem underlying the invention is also solved by a method which is based on the general concept of applying the injection pressure to the stored fuel fraction in its accumulator in order to prepare both the fuel fraction and the unfractionated fuel at the pressure level of the fuel for injection into the internal combustion engine.
In the method according to the invention, the accumulator which serves to store the fuel fraction serves as a condensation chamber of a fuel-processing apparatus which includes a vapor pump connected at the suction side to an evaporation chamber containing liquid fuel and at the vapor side to the condensation chamber, that is the accumulator. This measure results in a further simplification, since additional transport lines between the condensation chamber and the accumulator may be dispensed with. The pressure in the accumulator during the fractionating operation may be smaller than the injection pressure. With such a procedure, the vapor pump of the fuel-fractionating device may be dimensioned to be relatively small. In addition, the fractionating operation then requires less energy.
The evaporation chamber of the fuel-fractionating device may be filled with liquid fuel before the fractionating operation. A vapor space sufficient for the fractionating operation is however retained. The evaporation chamber is preferably filled with liquid fuel by generating a vacuum in the evaporation chamber, the evaporation chamber being connected to the fuel tank, so that the fuel is drawn out of the fuel tank into the evaporation chamber. With this procedure, the vapor pump, which is present anyway, may at the same time be used for delivering the liquid fuel from the fuel tank into the evaporation chamber.
Especially advantageous is an embodiment in which a flushing operation is carried out during the filling of the evaporation chamber with fuel. By the flushing operation the liquid fuel contained in the evaporation chamber is exchanged for the liquid fuel from the fuel tank. By means of this measure, xe2x80x9coldxe2x80x9d fuel of a preceding fractionating operation is exchanged for xe2x80x9cfreshxe2x80x9d fuel from the fuel tank, so that the fresh fuel has as high a proportion of the fuel fraction to be fractionated as possible for the following fractionating operation.
It is also possible to vent the condensation chamber before the fractionating operation. By means of this venting, air which has collected in the condensation chamber, that is in the accumulator, is drawn off and replaced by a gaseous fuel fraction. The effectiveness of the fractionating operation is increased as a result.
In an alternative embodiment of the method, liquid is taken from the bottom of the storage tank in a certain amount or for a certain amount or for a certain time before the fuel fraction is supplied to the engine. In this way, condensed water which may have formed in the storage tank and collected at the bottom thereof is removed before fuel injection is initiated. As a result, the admission of condensed water to the internal combustion engine is avoided.
For improving the processing of the fuel, a gaseous fluid may be introduced into liquid fuel in the form of small bubbles during the fractionating of the fuel. This improves the efficiency of the fractionating process.
Preferred embodiments of the invention will be described below in greater detail on the basis of the accompanying drawings.
Preferred exemplary embodiments of the invention are shown in the drawings and explained in more detail in the following description.